1. Field of the Invention
This invention relates to a method of strengthening chemically glass articles and more particularly it is concerned with a method of strengthening glass articles by effecting an ion exchange reaction using an aqueous solution of mixed potassium salts.
2. Description of the Prior Art
It is known that the intrinsic strength of glass is very high, for example, more than 7,000 Kg/cm.sup.2 in the non-scratched state, i.e., "pristine glass", but, when the surface is bruised, the strength is substantially lowered, for example, to about 200 Kg/cm.sup.2. When glass articles are handled, for example, during inspecting, bottling, capping, packaging and shipment, the glass articles are brought into contact with each other and thus scratched or bruised, whereby their strength is substantially lowered.
Various proposals have hitherto been made in order to protect glass articles from scratching or bruising and to improve the strength thereof. For example, there has been proposed a method of increasing chemically the mechanical strength of glass by a so-called ion exchange method wherein ions A contained in the glass surface, such as sodium ions, are replaced with ions B having a larger ion radius, such as potassium ions. This chemical strengthening method has been classified into the so-called fused salt immersion method, for example, as disclosed in Japanese Patent Publication No. 49298/1972, in which a glass article is contacted with, for example, a potassium salt bath and the sodium ions are replaced with the potassium ions, and the so-called aqueous solution application method, for example, as disclosed in Japanese Patent Publication Nos. 6610/1973 and 28674/1965, in which an aqueous solution of a potassium salt is applied to the surface of a glass article by spraying or by another suitable procedure and the glass article is held at a temperature for a period of time, which are sufficient to effect the ion exchange which will produce a compressive stress layer.
The inventors have made various studies on the ion exchange methods by application of adhesion of aqueous solutions, based on the concept that, when the outer surface and inner surface of a glass article having a complicated shape such as a glass bottle are subjected to a chemical strengthening treatment, the ion exchange method by application of an aqueous solution is more feasible on a commercial scale than the ion exchange method by direct application of a fused salt to a glass article (which will hereinafter be referred to as "fused salt method") and consequently have found that the ion exchange method by application of an aqueous solution according to the prior art has still a number of disadvantages. For example, the invention described in Japanese Patent Publication No. 28674/1965 has the disadvantage that, in order to strengthen chemically a glass article by the ion exchange method by application of an aqueous solution, it is necessary to add an inert aggregate carrier such as ochre to the ion exchanging agent and, therefore, many labours are required for removing the adhered substance from the glass article chemically strengthened. On the other hand, the invention disclosed in Japanese Patent Publication No. 6610/1973 is free from the above described disadvantage, but has other disadvantages that analysis of potassium salts as an ion exchanging agent is not completely effected and, when an aqueous solution of a potassium salt as an ion exchanging agent is applied to a glass surface by spraying, the aqueous solution tends to be hydrolyzed to be alkaline and to etch the glass surface, while the flowing-off phenomenon of the fused potassium salt adhered to the glass surface at the ion exchange temperature cannot be prevented.
In Japanese Patent Publication No. 4191/1972, there has been proposed for the purpose of making more adhesive an ion exchanging agent to a glass surface and preventing the ion exchanging agent from flowing off a process for the production of a glass article having a high strength, which comprises applying to a glass surface containing monovalent cations such fused mixed salts as consisting of at least two kinds of salts, forming a paste of a solid phase in a liquid phase through solidification of a part thereof by cooling at a temperature range of lower than the strain point of the glass and containing at least one monovalent cation having a larger radius in the liquid phase of the paste than the cation in the glass, and heating at a temperature of lower than the strain point of the glass for a desired period of time while holding the mixed salts on the glass surface as a coating layer of the above described paste, thereby forming a compressive stress layer on the glass surface. This technique can favourably be assessed as to prevention of the flowing-off phenomenon of an ion exchanging agent at an ion exchange temperature, but has disadvantages that not only the working is accompanied with dangers because of using fused mixed salts as the ion exchanging agent, but also a glass article must be preheated at a relatively high temperature resulting in a difficulty of working because the glass article tends to break if there is a large temperature difference between the glass article and fused salt, although the fused salt adheres fast to the glass article due to the temperature difference.
Since general glass containers, in particular, glass bottles are subjected to severe conditions such as brushing by means of a bottle washing machine before filling, processing with detergents, etc., the inner surface of the glass bottle tends to be scratched, resulting in marked lowering of the strength. In such cases, therefore, it is impossible to keep the strength by strengthening the outer surface of the glass bottle only and it is thus essential to strengthen the outer surface and inner surface of the glass bottle by the ion exchange method. Of the above described known methods, however, the aqueous solution application method as disclosed in, for example, Japanese Patent Publication Nos. 6610/1973 and 28674/1965 and the fused salt method as disclosed in Japanese Patent Publication No. 4191/1965 are not suitable for the ion exchange treatment of both the outer surface and inner surface of a glass article. In the former technique, there is no technical disclosure as to the ion exchange strengthening treatment of the inner surface of a glass container such as a glass bottle and in the latter technique, there is a technical disclosure as to the ion exchange strengthening treatment of both the outer surface and inner surface of a glass container, but, as described above, the working or operation is accompanied with some dangers because of using a fused salt bath and the handling of glass containers at a relatively high temperature results in complication of apparatus used therefor and difficulties in the operation.